The present invention relates to N-phosphonomethyliminodiacetic acid (xe2x80x9cPMIDAxe2x80x9d) of the formula (I): 
and to N-phosphonomethylglycine. Compound I is an important intermediate in the formation of N-phosphonomethylglycine (xe2x80x9cglyphosatexe2x80x9d), which is a translocated, postemergence, broad spectrum herbicide.
Glyphosate has heretofore been prepared by a variety of oxidations of PMIDA. For example, U.S. Pat. No. 3,954,848 discloses the production of glyphosate by the acid catalyzed oxidation-hydrolysis of PMIDA. Specifically, PMIDA is mixed with water and an acid and is heated to an elevated temperature. An oxidizing agent such as hydrogen peroxide is then added to convert the PMIDA to glyphosate, which is then isolated by precipitation. U.S. Pat. No. 3,969,398 discloses the oxidation of PMIDA to glyphosate employing a molecular oxygen-containing gas such as air, oxygen, oxygen diluted with helium, argon nitrogen or other inert gases, oxygen-hydrocarbon mixtures, etc., and employing activated carbon as the catalyst. U.S. Pat. No. 4,147,719 discloses the production of certain mono- and di-salts of glyphosate in a single aqueous reaction system by oxidizing a salt of PMIDA with a molecular oxygen-containing gas in the presence of platinum supported on an activated carbon substrate. The oxidation reaction is conducted at super-atmospheric pressures in the range of 1.5 to 5 kg/cm2 or higher. U.S. Pat. No. 4,898,972 discloses the production of glyphosate by the oxidation of PMIDA using a salt of cobalt or manganese in the presence of bromide ions. U.S. Pat. No. 4,002,672 discloses the production of glyphosate and salts thereof by the acid catalyzed hydrolysis of PMIDA. PMIDA is contacted with a strong acid having a pKa of less than 2.2, at an elevated temperature so as to cause the decomposition or hydrolysis of the phosphonomethyl iminodiacetic acid into N-phosphonomethyl-glycine and other decomposition products. U.S. Pat. No. 3,954,898 discloses how a large variety of oxidizing agents, including hydrogen peroxide, can be used to oxidize PMIDA to glyphosate in the presence of acetic or stronger acid, at 70-100xc2x0 C. U.S. Pat. No. 4,696,772 discloses how the activity of the activated carbon catalyst can be enhanced by first removing oxides of carbon from the surface of the carbon.
As can be seen from the foregoing, phosphonomethylglycine can be made by oxidizing phosphonomethyliminodiacetic acid by a myriad of techniques. However, all but a few of these produce an effluent that contains formaldehyde and toxic metals and/or strong acids. The use of very strong acid solutions at elevated temperatures necessitates the use of corrosion resistance equipment. Moreover, some of these processes must be run under very dilute conditions, requiring large amounts of energy to concentrate the solutions to allow the glyphosate product to be recovered in an economically attractive yield.
Phosphonomethylglycine also may be prepared by oxidizing PMIDA with an oxygen-containing gas in the presence of a noble metal co-catalyst for oxidizing by-product formaldehyde or formic acid. Oxidation of PMIDA in the presence of the noble metal but in the absence of activated carbon does not produce any product, see U.S. Pat. No. 3,950,402. The drawback to the noble metal catalysts is that they are very expensive and must be protected from rapid amine deactivation by either coating them with a polymer as disclosed in U.S. Pat. No. 4,579,689, or encapsulating them in a microcrystalline inorganic matrix, as disclosed in U.S. Pat. No. 4,582,650.
In accordance with U.S. Pat. No. 4,147,719, PMG salts may be prepared directly from a salt of PMIDA using an oxygen-containing gas in the presence of a noble metal oxidation catalyst, namely, platinum, supported on an activated carbon substrate. Again, though conversions are high, so is the cost of the catalyst. Thus, extreme care must be taken not to lose any catalyst during work-up of the product. The use of the noble metal catalyst is said to provide concurrent oxidation of the formaldehyde co-product as it is formed, which significantly reduces the time and expense otherwise required to remove this co-product from the final reaction product. The ""719 patent teaches that activated carbon catalysts alone do not provide for this concurrent oxidation of formaldehyde, and in fact cause increased production of undesirable by-products.
It is therefore an object of the present invention to provide a process for preparing phosphonomethylglycine that eliminates the drawbacks of the prior art processes.
It is a more specific object of the present invention to provide a process for preparing phosphonomethylglycine that eliminates the deleterious effluent and minimizes the cost of the catalyst.
The problems of the prior art have been overcome by the present invention, which provides a process for the preparation of phosphonomethylglycine in nearly quantitative yields, while simultaneously oxidizing the toxic by-product formaldehyde to formic acid and CO2 or salts thereof. In general terms, the process of the present invention eliminates the use of the noble metal co-catalyst and its concomitant problems. The oxygen-containing gas is replaced with the much more chemically active hydrogen peroxide, which in combination with activated carbon, oxidizes the PMIDA to PMG in very high yield, and oxidizes formaldehyde by-product to formate and CO2, leaving a no-longer toxic effluent that can be readily biodegraded. The rate of oxidant addition is carefully controlled and monitored.